It has been previously demonstrated that a critical control in the translation of mRNAs is at the level of phosphorylation of the eukaryotic initiation factor 2-alpha (eIF-2.alpha.). Two protein kinases have been shown to regulate initiation of translation by phosphorylation of this factor. The hemin-controlled repressor of protein synthesis (HCR) has been studied in reticulocytes and is activated by various stimuli including hemin deprivation and heat treatment. The double-stranded RNA activated inhibitor (DAI) is induced by interferon and its activity is dependent on double stranded RNA. It has previously been thought that phosphorylation by these kinases results in a general suppression of translation of cellular mRNAs. Several viruses encode specific regulatory mechanisms which can circumvent the translation inhibition imposed by activation of these kinases. One well studied example is adenovirus where a specific RNA gene product, the adenovirus associated RNA (VA RNA), can block the DAI kinase. Upon viral infection, the host cell induces and activates the DAI kinase as part of its antiviral response. Adenovirus can circumvent this antiviral response by the expression of the VA gene which directly blocks the kinase activity. Adenovirus mutants in the VA gene produce functional mRNAs but they are not translated as a result of the DAI kinase activation. In previous work we observed in a similar manner that transfected COS monkey cells exhibit suppressed translation and that the inefficient translation is restricted to mRNAs derived from the plasmid DNA. No translation suppression was observed on host mRNAs. Thus, there appeared to be a specificity in the translational block. The reason for this specificity is not known.
The research resulting in the present invention involved studying the effect of expression of eIF-2.alpha. at high levels and by expression of modified forms of eIF-2.alpha. which may not be susceptible to phosphorylation. The results of these studies provide (1) insight into the potential importance of translational control and (2) eucaryotic expression systems permitting the expression of foreign proteins at high level in animal cells.